Plasma torch assemblies



ZLE U LQQ Oct. 27, 1970 I P. MITCHELL 3,535,885

PLASMA TORCH ASSEMBLIES Filed Oct. 20, 1966 3 Sheets-Sheet 1 FIG! Oct.27, 1970 P. MITCHELL PLASMA TORCH ASSEMBLIES 3 Sheets-Sheet 2 Filed Oct.20, 1966 FIG FIG?

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Oct. 27, 1970 P. MITCHELL PLASMA TORCH ASSEMBLIES 3 Sheets-Sheet 5 FiledOct. 20. 1966 FIGEB FIGS) United States Patent 3,536,885 PLASMA TORCHASSEMBLIES Peter Mitchell, Birmingham, England, assignor to AssociatedElectrical Industries Limited, London, England, a British company FiledOct. 20, 1966, Ser. No. 588,172 Claims priority, application GreatBritain, Oct. 25, 1965, 45,096/ 65 Int. Cl. B23k 9/00 US. Cl. 219-121 18Claims ABSTRACT OF THE DISCLOSURE The operation of a plasma torch as analternating current supply at power frequency while obtaining thebenefit of low electrode wear normally associated with DC. operatedtorches by arranging that an arc is formed, and plasma forming gasconverted to plasma, only in similar halfcycles. This is achieved bypassing a pilot plasma into the plasma forming chamber and bycontrolling the composition and rate of flow of the plasma forming gasrelative to the pilot plasma such that the torch operates as arectifier. A plurality of torches can be interconnected to operate oneither similar or opposite half-cycles of the supply voltages.

Plasma torches which are so constructed as to operate from either a DC.source or a high frequency A.C. source are known but it is inconvenientto have to supply these special sources of electrical energy and it isan object of the present invention to provide a plasma torch which canbe operated from an alternating voltage source of mains frequency.

According to the present invention a plasma torch comprises having two.electrodes which provide inlet and outlet ports respectively for thechamber, means for passing plasma-forming gas into the chamber andbetween the electrodes and means for generating a pilot plasma which ispassed through the chamber by way of the inlet and outlet ports wherebya unidirectional current flows between the electrodes by way of saidpilot plasma when a power frequency alternating voltage is appliedtherebetween.

One of the advantages of such torches is that the electrodes have thelow-wear characteristics usually associated with plasma torches whichare operated from a DO.

source.

The pilot plasma projected into the chamber through the inlet portelectrically couples the two electrodes allowing a current to flowbetween them from the alternating source. The electrode which providesthe inlet port is heated, when in use, by the pilot plasma passingtherethrough and the relatively cold plasma forming gas flowing into thechamber cools the electrode which provides the outlet port to a greaterextent than it cools the other electrode. Furthermore by arranging forthe plasma forming gas conveniently nitrogen which is directed into thechamber to have a much greater electrical breakdown potential than thatof the gas, conveniently argon, from which the pilot plasma is formed,it can be arranged for the breakdown voltage in the direction from theoutlet port electrode to the inlet port electrode to be much higher thanthe breakdown voltage in the reverse direction. These factors enable aunidirectional current to flow between the electrodes. The direction ofelectron flow is from the electrode which provides the inlet port to theother electrode i.e. current flows during the half cycle of the appliedvoltage when the electrode which provides the outlet port is positivewith respect to the electrode which provides the inlet port.

The pilot plasma which is passed through the chamber may be generated byany convenient means from a DC.

3,536,885 Patented Oct. 27, 1970 ice reverse polarity DC). or AC. orhalf wave A.C. supply. When a half wave A.C. supply is employed it isessential that the plasma is generated at the periods of time whenconduction is required between the electrodes of the torch. The pilotplasma is generated between a pair of electrodes which may be completelyseparate from the electrodes of the torch but alternatively one of theelec trodes between which the pilot plasma is generated may be an axialextension of, or the reverse end of, the electrode which provides theinlet port of the chamber.

In order that the invention may be more readily understood, it will nowbe described, by way of example with reference to the accompanyingdrawings in which:

FIG. 1 is a sectional side elevation of a plasma torch in accordancewith one embodiment of the invention, showing the power suppliesthereto;

FIGS. 2 and 3 show in diagrammatic form alternative electricalconnections to the torch when used to heat an electrically conductivematerial by transferred flame operation;

FIGS. 4 and 5 show in diagrammatic form alternative electricalconnections to an assembly comprising a pair of plasma torches;

FIGS. 6 and 7 show in diagrammatic form alternative electricalconnections to a pair of plasma torches to bring about transferred flameoperation;

FIG. 8 shows in diagrammatic form the electrical connections to a deviceemploying four plasma torches to bring about transferred flame operationthereof, from a single phase supply; and

FIG. 9 shows in diagrammatic form the electrical connections to a deviceemploying six plasma torches to bring about transferred flame operationfrom a three-phase electrical supply.

Referring particularly to FIG. 1, a plasma torch 1 comprises a pair ofconductive electrodes 2 and 3 which are separated by an annular body ofelectrically insulating material 4 and which together define a generallycylindrical chamber 5. The electrodes 2 and 3 each have an openingtherein which openings are in axial alignment and provide an inlet port6 and an outlet port 7 respectively to the chamber. Electrode 2 isprovided with a hollow cylindrical part 8 manufactured from a refractorymetal i.e. tungsten and which projects into the chamber 5 with the boreof the part in alignment with the opening in the electrodes. Electrode 3is of refractory metal or copper and both of the electrodes haveprovision for liquid cooling in the vicinity of the opening therein.

A gas inlet port 9 is provided through the wall of the electrode 3 intothe chamber and the port is inclined almost tangentially to the innersurface of the wall of the cylindrical chamber so that plasma forminggas injected under pressure through the port into the chamber swirlsround the chamber in a vortex about the axis of the inlet and outletports 6 and'7.

Electrode 2 is electrically insulated from a further electrode 10 by anannular body 11 of insulating material. The electrodes 2 and 10 and thebody 11 define a further chamber 12 which is of generally cylindricalform and in communication with the chamber 5 by way of the port 6. Arodlike extension 13 projects from the electrode 10 into the chamber 11in axial alignment with the ports 6 and 7. The extension 13 may be oftungsten and with the electrode 2 constitutes a pilot plasma generator.Plasma forming gas is injected under pressure into the chamber throughan inlet port 14 so as to swirl around the chamber. The electrode 10 maybe liquid cooled.

To operate the assembly plasma forming gas, which conveniently may beargon, is injected into chamber 12 through the port 14 and aunidirectional voltage conveniently from a rectifier 15 fed from atransformer 16 is applied to terminals on the electrodes 10 and 2 withelectrode 10 negative with respect to electrode 2. An electric arc isset up between the electrodes and the plasma forming gas passing throughthe arc is converted to a plasma which streams through the opening inelectrode 2 into the chamber 5. A mains frequency alternating voltagefrom a transformer 17 is applied between terminals on electrodes 2 and 3and the pilot plasma provides a conductive path between theseelectrodes. An arc extends between the outer tip of extension part 8 andthe adjacent surface of the electrode 3, and plasma forming gas injectedinto the chamber through port 9 constricts the arc and forces it partway into the outlet port 7. The plasma forming gas which convenientlymay be nitrogen passes through the arc and is converted into a plasmawhich reinforces the pilot plasma leaving the chamber 5 through theoutlet port 7. By suitably adjusting the spacing between the electrodes2 and 3 and the composition and rate of flow of the plasma forming gasesinjected into the chambers 5 and 12, the torch can be made to operate ina self rectifying manner. Although the pilot plasma may flowcontinuously into the chamber 5 an arc is only formed between electrodes2 and 3 during the half cycles of the supply applied between theseelectrodes when electrode 3 is positive with respect to electrode 2,i.e. electrode 2 acts as the cathode and electrode 3 as the anode.During the reverse half cycles there is no current flow between theelectrodes.

The plasma produced by the torch shown in FIG. 1 is usually used as aheating source but by modifying the electrical connections to the torchas shown in FIGS. 2 and 3 the plasma leaving the torch can be used as aconductive column between the electrodes of the torch and anelectrically conductive work-piece or charge. In FIGS. 2-9 the plasmatorch shown in FIG. 1 is indicated diagrammatically by the threeelectrodes 13, 2 and 3 and the electrical supply for producing the pilotplasma is not shown.

Referring to FIG. 2, an alternating mains frequency electrical supplyfrom transformer 17 is connected between the electrodes 2 and 3 and afurther alternating supply of the same mains frequency from atransformer 21 is connected between the electrode 2 and a conductivecharge 22 in a furnace indicated by reference numeral 23. The polarityof the two electrical supplies are arranged so that the charge ispositive 'with respect to the electrode 2 at the same time thatelectrode 3 is positive with respect to electrode 2 and during the halfcycles when this occurs the plasma leaving the torch is used to passcurrent into the charge. The current flow in the two electrical circuitsis shown by the full arrowed lines. If as shown in FIG. 3, the supplyfrom the transformer 21 is connected between the electrode 3 and thecharge in the furnace with the polarity of the windings of transformers17 and 21 arranged such that the charge is negative with respect toelectrode 3 when electrode 2 is negative with respect to electrode 3then half cycles of electrical energy flow between the electrodes 2 and3 and 1 half wave current flows through the plasma from the charge tothe electrode 3. It will be seen that to draw a full-wave current fromthe supply connected between the electrodes 2 and 3 it is necessary toemploy two torches and operate them alternately. The electricalconnection for such an assembly is shown in FIG. 4. Two torches 40 and41 are connected together electrically with the electrode 2 of torch 40connected to the electrode 3 of torch 41 and the electrode 2 of torch 41and the elec trode 3 of torch 40 are similarly connected together. Theelectrical supply from a transformer 42 is connected between theelectrodes 2 and 3 of torch 41. During the half cycles of the supplywhen the electrode 3 of torch 41 is positive with respect to theelectrode 2 of the torch, electrons flow between them in the directionindicated by the full arrowed line and current is drawn from the supply.During the alternative half cycles when the electrode 2 of this torch ispositive with respect to the electrode 3 of the torch, no electrons flowbetween them. However during these latter half cycles the electrode 3 oftorch 40 is positive with respect to its electrode 2 and half cycles ofcurrent are drawn from the supply and flow bet-ween these electrodes inthe direction indicated by the broken arrowed line.

FIG. 5 shows an alternative arrangement in which the electrode 2 of twotorches 50 and 51 are connected to the respective outer ends of a centretapped secondary winding of a transformer 52 and a centre tap isconnected to the electrode 3 of both torches. During the half cycles ofthe supply when the electrode 3 of torch 50 is positive with respect tothe electrode 2 thereof current flows therebetween in the directionshown by the full arrowed line. During the alternate half cycles of thesupply when the torch 50 is not conducting, torch 51 conducts with thecurrent flowing in the direction indicated by the broken line.

The arrangement illustrated in FIG. 6 is a combination of the featuresof the assemblies shown in FIGS. 3 and 4 to enable a continuoustransferred arc to a conductive charge 22 to be made. The same result isobtained by the assembly shown in FIG. 7 which combines the feature ofthe torch and assembly shown in FIGS. 2 and 5 respectively.

FIG. 8 illustrates a device for transferred-arc operation but withoutthe necessity of applying an electrical connection direct to the charge.Two assemblies 81 and 82 each of the type illustrated in FIG. 4 and eachconsisting of two torches a and b have a further mains frequencyelectrical supply from a transformer 83 applied between the electrode 3of torch 81b and the electrode 3 of torch 82a. The streams of plasmafrom the torches to the conductive charge 22 serve as conductive columnsbetween the torches and the charge and a full wave current is taken fromthe secondary winding of transformer 83. During one set of alternatehalf cycles the electron flow is as indicated by full arrowed lines andduring the other set of alternate half cycles the electron flow is asindicated by broken arrowed lines.

FIG. 9 shows a device by which three plasma torch assemblies 90, 91, and92 each consisting of two torches can be operated with transferred arefrom a three-phase supply. The two torches in each assembly have theirnoncorresponding main electrodes connected together as described inconnection with FIG. 4 and the electrode 2 of the corresponding torch ofeach assembly is connected to a separate phase of the three phasesecondary windings of a three phase transformer 93. The full arrowedlines indicate the electron flow during one half cycle of one phase ofthe supply and the broken arrowed lines indicate the direction ofelectron flow during the reverse half of that cycle. Although the phasesof the transformer 93 are shown connected in star, a delta connectioncan be employed provided that the phase of the voltage between the mainelectrodes is maintained the same as the corresponding transfer voltage.There is no direct electrical connection between the conductive charge22 and any of the torches or the various electrical supplies.

What I claim is:

1. A method of operating a plasma torch from a source of alternatingvoltage at power frequency including the steps of applying said voltagewithout prior rectification between a pair of electrodes of the torch,continuously passing a pilot plasma between said electrode, passing aplasma forming gas between said electrodes with the composition and rateof bow of the plasma forming gas selected in relation to the electrodesto cause the torch to operate as a rectifier, such that arcs are struckbetween the said electrodes only during similar half cycles of saidvoltage but not during the opposite half-cycles, to convert said plasmaforming gas to plasma only during said similar half-cycles.

2. A method of operating a plasma torch from a source of alternatingvoltage at power frequency, as claimed in claim 1 in which saidelectrodes provide an inlet and an outlet port respectively, of achamber into which said pilot plasma and said plasma forming gas aredirected and holding said electrode which provides the outlet port at alower operating temperature than that providing the inlet port wherebysaid arcs are struck only during the half cycles of the alternatingvoltage when said electrode providing the outlet port is positive withrespect to the other electrode.

3. A method of operating a plasma torch as claimed in claim 2, in whichsaid plasma forming gas introduced into the chamber serves to cool theelectrode providing the outlet port to a greater extent than it coolsthe electrode providing the inlet port.

4. A method of operating a plasma torch from a source of alternatingvoltage at power frequency as claimed in claim 1 in which saidelectrodes provide an inlet port and an outlet port respectively of achamber, said pilot plasma is directed into said chamber through saidinlet port, said pilot plasma is formed from a gas having a much lowerelectrical breakdown potential than that of the plasma forming gaspassed between the electrodes and said arcs are struck only during thehalf cycles of the alternating voltage when said electrode providing theoutlet port is positive with respect to the other electrode.

57 A method of operating a plasma torch from a source of alternatingvoltage at power frequency as claimed in claim 1 in which one of saidelectrodes provides an inlet port and the other electrode provides anoutlet port of a chamber, said pilot plasma is directed into saidchamber through said inlet port, said pilot plasma is formed from argon,said plasma forming gas which is passed between said electrodes isnitrogen and said arcs are struck only during the half cycles of thealternating voltage when said electrode providing the outlet port ispositive with respect to the other electrode.

6. A method of operating a plasma torch as claimed in claim 1 to heat anelectrically conductive material in which the plasma generated in saidtorch is directed onto said conductive material and a further source ofalternating voltage at power frequency is connected between saidconductive material and one of the electrodes of the torch such thathalf waves of current flow between said electrode and the conductivematerial.

7. The method of claim 1, wherein said plasma forming gas is introducedin an amount sufficient to maintain the second electrode at atemperature substantially below that of the first electrode.

8. The method of claim 1, wherein the plasma forming gas passed betweenthe said electrodes has a higher electrical breakdown potential than theplasma forming gas used to form the said pilot plasma.

9. A plasma torch assembly comprising two similar plasma torches eachcomprising a pilot plasma chamber, a main plasma forming chamber, firstand second spaced apart electrodes which are apertured to provide aninlet and an outlet port respectively for said main plasma formingchamber and which are electrically insulated from one another, and firstand second terminal means in electrical contact one with each of theelectrodes for enabling an alternating voltage to be applied thereto,means for introducing a gas having a low electrical breakdown potentialinto the pilot plasma chamber for the generation of a pilot plasmaintroduceable into the main plasma forming chamber through the inletport, means for introducing plasma forming gas of higher electricalbreakdown potential under pressure between the electrodes in said mainplasma forming chamber of each said torch, such that the two torches areoperable as rectifiers to convert the plasma forming gas to plasma inalternate half cycles of said voltage, and electrical connectionsbetween the first electrode of each torch and the second electrode ofthe other torch.

10. A plasma torch assembly comprising two similar plasma torches eachhaving a pilot plasma chamber, a main plasma forming chamber, first andsecond spaced apart electrodes which are apertured to provide an inletand an outlet port respectively for said main plasma forming chamber,and which are electrically insulated from one another, means forintroducing a gas having a low electrical breakdown potential into thepilot chamber for the generation of a pilot plasma introduceable intothe main plasma forming chamber through the inlet port, means forintroducing a plasma-forming gas of higher electrical breakdownpotential under pressure between the electrodes in said main plasmaforming chamber of each torch, electrical connections between the firstelectrodes of each torch and the second electrode of the other torch,and terminal means for enabling an alternating voltage at powerfrequency but of different polarity to be applied to the electrodes ofthe two torches to cause each torch to operate as a half-wave rectifierand its plasma forming gas to be converted to plasma only during thehalf-cycles of said voltage in which the second electrode is positivewith respect to the first electrode.

11. A plasma torch assembly according to claim 10, wherein each of thetorches is disposed so as to direct the plasma generated therein ontosaid conductive material through said outlet port and wherein there areincluded means for applying an alternating voltage at power frequencybetween said conductive material and the second electrode of one of thetorches.

12. A plasma torch assembly comprising tWo similar plasma torches eachcomprising a pilot plasma chamber, a main plasma forming chamber, firstand second spaced apart electrodes which are apertured to provide aninlet and an outlet port respectively for the main plasma formingchamber, and which are electrically insulated from one another, firstand second terminal means in electrical contact one with each of theelectrodes, means for introducing a gas having a low electricalbreakdown potential into the pilot chamber for the generation of a pilotplasma introduceable into the main plasma forming chamber through saidinlet port, means for introducing plasma forming gas of higherelectrical breakdown potential under pressure between the electrodes insaid main plasma forming chamber, and a transformer having a tappedsecondary winding opposite ends of which are connected each to one of apair of similar electrodes of the torches and the tapping of which isconnected to the other pair of similar electrodes of the two torches,means for applying an alternating voltage at power frequency to theprimary winding of the transformer to cause electric arcs to be struckbetween the electrodes of each torch in turn during the half-cycles inwhich the electrode of the torch which is connected to the tapping ispositive with respect to the other electrode of the torch, and therebyto convert the respective plasma forming gas to plasma during saidhalfcycles.

13. A plasma torch assembly for heating an electrically conductivematerial comprising two similar plasma torches each comprising a pilotplasma chamber, a main plasma forming chamber, first and second spacedapart electrodes which are apertured to provide an inlet and an outletport respectively for the main lasma forming chamber, and which areelectrically insulated from one another, first and second terminal meansin electrical contact one with each of the electrodes, means forintroducing a gas having a low electrical breakdown potential into thepilot chamber for the generation of a pilot plasma introduceable intothe main plasma forming chamber through said inlet port, means forintroducing plasma forming gas of higher electrical breakdown potentialunder pressure between the electrodes in said main plasma formingchamber, and a transformer having a primary winding arranged to beconnected to a source of alternating voltage at power frequency andsecondary windings connected so as to apply between the electrodes ofthe two torches alternating voltages of opposite polarity such that theelectrodes of the two torches act as a rectifier to convert therespective plasma forming gases to plasma only during differentalternate half-cycles, a further transformer having a primary windingarranged to be connected to a source of alternating voltage at powerfrequency and a tapped secondary winding opposite ends of which areconnected each to one of a pair of similar electrodes of the two torchesand the tapping of which is connectable to the said conductive material.

14. Apparatus for heating electrically conductive material comprising apair of plasma torch assemblies, each of which assemblies comprises twosimilar plasma torches each having a pilot plasma chamber, a mainplasma-forming chamber, first and second spaced apart electrodesapertured to provide an inlet and an outlet port respectively for saidchamber, and electrically insulated from one another, first and secondterminals connected to the first and second electrodes, means forintroducing a gas having a low electrical breakdown potential into thepilot chamber for the generation of a pilot plasma introduceable intothe main plasma forming chamber through said inlet port, means forintroducing plasma forming gas of higher electrical breakdown potentialunder pressure between the electrodes in said main plasma formingchamher, the terminals of one of the torches of each assembly beingconnected to a respective source of alternating voltage at powerfrequency, electrical connections being provided between the firstelectrode of one torch of each assembly to the second electrode of theother torch of the assembly such that the two torches are operable toconvert the respective plasma forming gases to plasma in alternatehalf-cycles of the applied voltage, and a transformer having itssecondary winding connected between similar electrodes of two torchesone from each assembly, and its primary winding connected to a source ofalternating voltage at power frequency.

15. Apparatus for heating electrically conductive material comprisingthree plasma torch assemblies each of which comprises two similar plasmatorches each having first and second spaced apart electrodeselectrically insulated from one another, first and second terminalsconnected to the first and second electrodes, means for introducing agas having a low electrical breakdown potential into the pilot chamberfor the generation of a pilot plasma introduceable into the main plasmaforming chamher through said inlet port, means for introducing plasmaforming gas of higher electrical breakdown potential under pressurebetween the electrodes in said main plasma forming chamber, andelectrical connections being provided between the first electrodes ofeach torch of each assembly, and its second electrode of the other torchof the assembly such that the two torches are operable to convert therespective plasma forming gases to plasma in alternate half-cycles ofthe applied voltage, and corresponding electrodes from each of the threeassemblies being connected to respective secondary windings of athree-phase transformer, the primary winding of which is connected to athree-phase alternating voltage source at power frequency.

16. A plasma torch comprising means defining a chamber, two spaced apartelectrodes which provide inlet and outlet ports for the chamber, saidelectrodes including first and second electrodes which are electricallyinsulated from one another, means for introducing lasma forming gasunder pressure into the chamber between said first and second electrodesso that the gas flows between said electrodes, means for generating apilot plasma which continuously enters the said chamber via said inletport, terminal means for enabling and alternating voltage at powerfrequency to be applied between the two electrodes, and means foroperating the torch such that the torch opcrates as a rectifier and arcsare formed and the plasma forming gas converted to plasma only duringsimilar half-cycles of said alternating voltage.

17. A plasma torch according to claim 16 wherein the means for operatingthe torch so that the torch operates as a rectifier comprises means forintroducing said plasma forming gas such that its electrical breakdownpotential is greater than the plasma forming gas used to form the saidpilot plasma.

18. A plasma torch according to claim 16, wherein the means foroperating the torch so that the torch operates as a rectifier comprisesmeans for introducing said plasma forming gas under pressure between theelectrodes in the main plasma-forming chamber to be expelled from theoutlet port, in an amount sufficient to maintain the second electrode ata temperature substantially below that of the first electrode.

References Cited UNITED STATES PATENTS 3,248,513 4/1966 Sunnen 2191213,373,306 3/1968 Karlovitz 219- 2,756,311 7/1956 Persson et al 219-RALPH F. STAUBLY, Primary Examiner J. G. SMITH, Assistant Examiner US.Cl. X.R. 21975

